Synthesis of nanofibers of polyaniline and substituted derivatives

ABSTRACT

Novel, simple methods are presented directed to the synthesis of nanofibers of polyaniline and substituted derivatives. The production of these fibers is achieved via various methods by controlling the concentration of aniline monomer or substituted aniline derivatives or an oxidant in the reaction medium and maintaining said concentration at a level much lower than conventional polyaniline synthesis methods. Methods are disclosed relating to the use of a permeable membrane to control the release of a monomer and/or oxidant as well as a bulk polymerization method.

BACKGROUND

The present exemplary embodiments relates to the synthesis ofpolyaniline and its substituted derivatives. It finds particularapplication in conjunction with the synthesis of conductive polyanilinenanofibers, and will be described with particular reference thereto.However, it is to be appreciated that the present exemplary embodimentis also amenable to other like applications such as other conductive andsemiconductive polymers.

Electroconductive polymers have been subject to extensive research inrecent years. Polymers which show electrical conductivity due to thestructure of the polymeric chain may be used to replace metal conductorsand semiconductor materials in many applications. Significantapplications include providing a conductive pathway in circuits anddevices, displays, lighting, chemical, biological, environmental andmedical sensors, anticorrosive coatings, scaffolds for tissue growth,antistatic shielding (ESD) and electromagnetic shielding (EMI).

In the group of intrinsically electroconductive polymers, onetechnically promising polymer is polyaniline. Polyaniline has emerged asone of the most promising conducting polymers and can be used in avariety of applications, such as paint, antistatic protection,electromagnetic radiation protection, electro-optic devices such asliquid crystal devices (LCDs), light emissive displays, lighting andphotocells, transducers, circuit boards, chemical, biological,environmental and medical sensors, anticorrosive coatings, scaffolds fortissue growth, etc.

Polyaniline is one of a class of conductive polymers, which can besynthesized through either chemical polymerization or electrochemicalpolymerization. Polyaniline is conventionally prepared by polymerizingan aniline monomer. The nitrogen atoms of monomer units are bonded tothe para-carbon in the benzene ring of the next monomer unit. Inchemical preparation, bulk polymerization is the most common method tomake polyaniline. As has been previously reported, conventional bulkchemical synthesis produces granular polyaniline.

Polyaniline nanofibers have attracted much attention in the field ofnanotechnology. There have been recent reports of a variety of chemicalmethods used in order to obtain polyaniline nanofibers. These approachesinclude use of templates or surfactants, electrospinning, coagulatingmedia, interfacial polymerization, seeding, and oligomer-assistedpolymerization.

BRIEF DESCRIPTION

In the present application, a novel, simple method is introduced toprepare polyaniline nanofibers. In one embodiment, a permeable tubing ormembrane is used to steadily control the release of aniline monomer intoan oxidant solution (or vice versa) to form polyaniline nanostructures.After polymerization, polyaniline nanofibers are collected directlyoutside the tubing or inside the tubing without any further treatment toobtain free-standing nanofibers.

In a second embodiment, polyaniline nanofibers can be obtained fromconventional bulk chemical polymerization under careful control ofpolymerization conditions. This is accomplished by introducing anilinemonomer solution into an oxidant solution (or vice versa) andpolymerizing at very low concentrations.

In a third embodiment, there is provided a field effect device having anactive channel including a polyaniline or substituted polyanilinenanofiber network in contact with a source electrode and a drainelectrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a scanning electron micrograph (SEM) of polyaniline/CH₃SO₃ ⁻nanofibers made by the first described method deposited on Si-wafersubstrate with a thin layer coating of Au/Pd (scale bar=2 μm).

FIG. 1 b is a transmission electron micrograph (TEM) ofpolyaniline/CH₃SO₃ ⁻ nanofibers made by a first described methoddispersed in deionized water. (scale bar=500 nm).

FIG. 2 is XRD patterns of polyaniline nanofibers and Electrondiffraction (inset image (a)) of a polyaniline/CH₃SO₃ ⁻ nanofiber madeby made by the first described method (inset image (b)).

FIG. 3 is a UV/vis spectra of polyaniline nanofibers made by the firstdescribed method dispersed in deionized water after purification, afterdialysis with 0.1M NH₄OH_((aq)) and after dialysis with 0.5M HCl_((aq)).

FIG. 4 is an FTIR spectrum of polyaniline/CH₃SO₃ ⁻ nanofibers made bythe first described method showing five major vibration bands: 1574,1490, 1294, 1132 and 796 cm⁻¹.

FIG. 5 are Scanning electron micrograph (SEM) of polyaniline nanofibersmade by the first described method deposited on Si-wafer substrates.

FIG. 6 are transmission electron micrograph (TEM) of polyanilinenanofibers made by the second described method obtained in differentdopant acids.

FIG. 7 are SEM images of polyaniline nanofibers made by the seconddescribed method synthesized in different dopant acids.

FIG. 8 shows scanning electron micrograph (SEM) of polyaniline/CH₃SO₃ ⁻nanofibers obtained via made by the second described method (bulkpolymerization) at 24° C. (a) without mechanical stirring and (b) withmechanical stirring.

FIG. 9 are SEM images of polyaniline nanofibers made by the seconddescribed method made in different concentration of aniline.

FIG. 10 is a UV/vis absorption spectra of polyaniline/ClO₄ ⁻ nanofibersmade by the second described method.

FIG. 11 is an example of a sensor based on polyaniline nanofiber networkprepared by the methods described herein.

FIG. 12 is a graph showing the conductance change of nanofibers uponapplying gate voltages in a field effect device based on polyanilinenanofibers.

DETAILED DESCRIPTION

As stated above, the present exemplary embodiments are directed to thesynthesis of polyaniline nanofibers. The production of these fibers isachieved via various methods by controlling the concentration of anilinemonomer or an oxidant in the reaction medium and maintaining saidconcentration at a level much lower than conventional polyanilinesynthesis methods. Although not intended to be limiting, excellentresults are achieved with a concentration of monomer in a reactionsolution of 10 millimoles or less. This control can be accomplished byvarious methods.

In a first embodiment, aniline monomer dissolved in an aqueous acidsolution is separated from an aqueous oxidant/acid solution by apermeable membrane in a reaction chamber. The aniline monomer diffusesthrough the membrane at a controlled rate and is subsequentlypolymerized in the oxidant/acid solution according to known reactions.Alternately or in addition to diffusion of the aniline monomer, theoxidant can diffuse through the membrane. Polyaniline nanofibers willform and then precipitate out of aniline and oxidant solution and may besubsequently collected.

The permeable membrane may be any membrane through which the anilinemonomer and/or oxidant can diffuse or otherwise pass through. Thus,various types of cellulose or other finely porous materials may be usedas the membrane. Useful membranes may thus be made from, for example,regenerated cellulose, cellulose ester, or polyvinylidene difluoride.The arrangement of the membrane can vary depending on the size, shape,etc. of the reaction chamber, with the only provision being that it mustseparate the monomer from the oxidant.

In one specific embodiment, applicants have found that conventionalregenerated cellulose dialysis tubing provides excellent results in thatit adequately controls the diffusion of monomer(s) or oxidant(s) toenable the production of extremely fine polyaniline nanofibers. Thus, inthis embodiment, aniline monomer in solution is placed in dialysistubing, which is then sealed. The sealed tubing is then placed in areaction chamber (such as a beaker) containing an oxidant in an acidsolution. Alternately, the oxidant may be placed in the tubing with theaniline monomer in the reaction chamber.

In this embodiment, the pore size of the dialysis tubing may be changedto control the rate of diffusion of the aniline and/or the oxidant andthus its concentration in the reaction chamber. This control can be usedto customize the size and configuration of the resulting polyanilinenanofibers, as described below. Applicants have found that a regeneratedcellulose membrane (or tubing) with a molecular weight cut off (MWCO) ofabout 3500 to 60,000 provides excellent results. Nevertheless, othermembranes with larger or smaller pore sizes may be used. Thus, celluloseester membranes with MWCO of from 100 to 300,000 or polyvinylidenedifluoride membranes with MWCO of from 250,000 to 1,000,000 are alsosuitable exemplary materials.

Polyaniline produced according to the process of this invention may beprepared from the polymerization of unsubstituted aniline or asubstituted aniline monomer. In addition, dimers as well as oligomershaving up to eight repeating aniline or substituted aniline units mayalso be used in the various embodiments. As used herein, any generaldescription using the terms “aniline” is intended to refer to andencompass both substituted and unsubstituted aniline monomer, as well asdimers or oligomers thereof of up to eight units in length. Likewise,the term “polyaniline” is also intended to refer to and encompasspolymers of both substituted and unsubstituted anilines unlessspecifically noted.

Exemplary substituted aniline monomers include those having thefollowing formula:

wherein, R₁, R₂, R₃, R₄, R₅, and R₆ are independently selected from thegroup consisting of: hydrogen atom, alkyl, alkoxy, alkylsulfonyl,arylsulfonyl, halogen, alkoxycarbonyl, alkylhio, alkylsulfuryl,cycloalkyl, sulfonic, aryl or carboxylic substituted alkyl substituents.

Specific substituted anilines that may be amenable to the presentprocesses include 2-aminobenzenesulfonic acid, 3-aminobenzenesulfonicacid, orthanilic acid, o-toluidine, m-toluidine, o-anisidine,m-anisidine, as well as polyhalogen anilines such as 2-fluoroaniline,2-chloroaniline, 2-bromoaniline, 2-iodoaniline, 3-fluoroaniline,3-chloroaniline, 3-bromoaniline, and 3-iodoaniline. In addition, it maybe possible to use other monomers by modifying the disclosed processesincluding, for example, pyrrole, substituted pyrrole, thiophene,substituted thiophene and 3,4-ethylenedioxythiophene as well as the useof two or more monomers to produce a copolymer, such as aniline/pyrrole,aniline/touidine or aniline/anisidine. Specific nanofibers of bothpoly(-o-toluidine) and sulfonated polyaniline were successfully producedusing the present processes.

In an aqueous polymerization medium, any conventional protonic acid ormixtures thereof may be used in the present embodiments. Both inorganicand organic acids may be used including chiral acids. Such acids for usein the polymerization of aniline are known and include, but are notlimited to, protonic acids which can be used to form a complex with theaniline monomer and to make it possible for the aniline monomer to bedissolved in water. Exemplary acids include hydrochloric acid, hydrogenbromide, sulfuric acid, perchloric acid, nitric acid, phosphoric acid,phosphonic acid, trifluoromethanesulphonic acid, toluenesulphonic acid,dodecylbenzenesulphonic acid (DBSA), acetic acid, trichloroacetic acid,trifluoroacetic acid, formic acid, (1R)-(−)-10-camphorsulfonic acid,(1S)-(+)-10-camphorsulfonic acid (CSA),2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA), andmethanesulfonic acid (CH₃SO₃H), carboxylic acids, etc. It is alsopossible to use a mixture of these protonic acids. Also Lewis acids canbe used. The invention is not limited to the use of the above-mentionedacids.

The oxidative agent used in the process according to the presentembodiments may be any conventional oxidizer used in the polymerizationof aniline. Exemplary oxidizing agents include ammonium peroxydisulfate(APS), persulfated salts such as, potassium persulfate, perchloratedsalts such as potassium perchlorate, chlorinated salt such as potassiumchlorinate, iodonated salt such as potassium iodonate, chlorinated ironsuch as ferric chloride, halogenated metal acids such as chloroaurateacid, fuming sulfuric acid, and ozone, particularly from APS, K₂Cr₂O₇,KlO₃, FeCl₃, KMnO₄, KBrO₃, KClO₃, peracetic acid or hydrogen peroxide.The reduced oxidant may remain in the resulting polymer nanofibers, asfor example, iron or gold nanoparticles.

The polymerization temperature in the process of the present embodimentsmay vary within a range from −10 to 60° C. Similarly, the pH of thereaction solution is preferably maintained at a pH value of belowabout 1. However, nanofibers can also be produced at a pH value of 1 orabove.

As detailed above, the membrane is used to steadily control the releaseof aniline and/or oxidant into the other solution to form polyanilinenanostructures. In conventional bulk polymerization methods, the anilinemonomer is typically present in the reaction solution at a molarconcentration of about 0.3 to 0.6. In the specific embodiments hereinwherein the aniline diffuses through the membrane to react with theoxidant (thus making the oxidant solution the site of the reaction),there may be a much lower concentration of aniline in the reactionsolution, for example on the order of about 0.001 to about 0.01 M,preferably about 0.008 M. Alternately, if the oxidant is the speciesthat diffuses through the membrane (thereby making the aniline solutionthe site of reaction), then the concentration of oxidant in the reactionsolution may fall within the above ranges.

This low concentration is achieved in these embodiments by the slowdiffusion of aniline (or oxidant) across the membrane. Aniline can beused directly or dissolved in any acid solutions or in any organicsolvents with any concentrations. In the embodiment below, this lowconcentration is achieved by the introduction of much smaller amounts ofaniline into the reaction chamber. The amount of oxidant initiallypresent in solution prior to polymerization relative to the amount ofaniline initially present in solution is not critical, but applicantshave found that a preferred molar ratio of aniline to oxidant is 1:1. Ithas been found that stirring or otherwise agitating the reaction mixtureduring polymerization may be desirable in some instances to producenanofibers having specific characteristics.

in the first embodiment the characteristics of the resulting polymernanofibers (including diameter and morphology) can be controlled to acertain degree by the selection of acid to be added to the reactionmixture as well as the temperature at which the polymerization iscarried out and the inclusion of a surfactant.

In the first embodiment suitable surfactants that may be used in thereaction system include anion surfactants such as sodium dodecylsulfate,cation surfactants such as dodecyltrimethylammoniumbromide, and nonionicsurfactants such as Triton® X-100. When included, the concentration ofsurfactant in the reaction mixture may range from, e.g., 0.0001 M to 1M.

The resulting doped polymer can be dedoped with a base to produce anon-electroconductive polyaniline product (emeraldine base) withelectrical conductivity less then 10⁻⁵ S/cm, which can be re-doped witha suitable acid to produce an electroconductive polymer with the desiredproperties. By this dedoping and redoping process, it is possible tocontrol the electro-conductive properties of the polymer nanofiberscontinuously over the full range from that of an electrical insulator(conductivity <10⁻¹⁰ S/cm) to that of a semiconductor (conductivity˜10⁻⁵ S/cm) to that of a good conductor of electricity (conductivity ˜1S/cm) to that of a metal (conductivity >10⁺² S/cm).

In a second embodiment, bulk polymerization of aniline or substitutedaniline monomer is conducted at very low concentration of anilinemonomer. This is accomplished by introducing aniline monomer solutioninto an oxidant solution (or vice versa) and polymerizing at very lowconcentrations. Suitable concentrations may be tens of millimoles orlower and preferably from 0.001 to 0.01 M. Applicants have found thatthis low concentration allows the production of polyaniline nanofibers.Applicants have found that this low concentration coupled with theeffect of minimal or not stirring or agitating the reaction mixtureduring the polymerization, allows the production of polyanilinenanofibers of increased length. However, it has been found that stirringor otherwise agitating the reaction mixture during polymerization may bedesirable in some instances to produce nanofibers having specificcharacteristics.

In this second embodiment, a typical bulk polymerization reactionapparatus may be used. This typically consists of a reaction chamber,which in its simplest form may be a beaker. An aqueous solution ofprotonic acid, oxidant(s) and, if necessary, other agents are added intothe reaction chamber. Oxidant(s) can be dissolved in an acid solutionfor example in the mixing tank. A commonly used oxidant is APS. Alsoother oxidants can be used. Protonic acid makes the polymerizing mediumacidic, thereby making the polymerization reaction possible. Protonicacid also acts as a so-called dopant which donates the counter anion andforms a salt with the polyaniline base. Suitable acids are describedabove.

The actual polymerization takes place by feeding monomer(s), e.g.aniline into the process. Dissolved into a suitable medium, such as anaqueous acid solution, aniline is supplied to the reaction chamber.Depending on the temperature of the reaction mixture, the polymerizationtakes place over the course of several hours. While stirring istypically used in the polymerization of aniline and can be performed inthe present process, applicants have found that longer and less branchedfibers are possible if the mixture is subjected to minimal stirring orotherwise not agitated. Polymerized aniline precipitates to the bottomof the reaction chamber, which can then be collected and purified.

The amount of oxidant initially present in the reaction solution priorto polymerization relative to the amount of aniline initially present inthe reaction solution is not critical, with the initial molar ratio ofaniline to oxidant ranging from 50:1 or greater down to 0.02:1. Morepreferred molar ratios are from 10:1 to 0.1:1 and even 4:1 to 1:1. Aparticularly suitable molar ratio of aniline to oxidant is 1.5:1.

In the second embodiment the characteristics of the resulting polymernanofibers (including diameter and morphology) can be controlled to acertain degree by the selection of acid to be added to the reactionmixture as well as the temperature at which the polymerization iscarried out and the inclusion of a surfactant.

In the second embodiment suitable surfactants that may be used in thereaction system include anion surfactants such as sodium dodecylsulfate,cation surfactants such as cetyltrimethylammoniumbromide, and nonionicsurfactants such as Triton® X-100. When included, the concentration ofsurfactant in the reaction mixture may range from, e.g., 0.0001 M to 1M.

The polymer nanofiber networks so made can be used for chemical,biological, environmental or medical sensors.

The nanofiber networks also can be used as the active channel of a fieldeffect device. FIG. 12 is a graph showing the conductance change ofnanofibers upon applying gate voltages in a field effect device based onpolyaniline nanofibers in contact with source and drain electrodes. Thenanofiber network is in contact with a dielectric polymer which is incontact with a gate electrode. Application of a gate voltage of lessthen 20 volts results in modulation of the electrical conductivitybetween source and drain electrodes.

The following examples are provided for purposes of describing thepreferred embodiments. They should not be considered limiting of theinvention.

EXPERIMENTAL Example 1 Using Permeable Tubing

Aniline (Aldrich) was distilled under vacuum before use. Ammoniumperoxydisulfate (APS; 99.99%, Aldrich) and methanesulfonic acid(CH₃SO₃H; 99.5%, Alfa Aesar) were used directly as received withoutfurther purification. Spectra/Por Dialysis Tubing, Regenerated Cellulose(MWCO 3500 and MWCO 12k-14k) and Spectra/Por Closures were purchasedfrom Spectrum Laboratories, Inc.

Aniline (150 mg) was dissolved in 3 mL of 1M methanesulfonic acid(CH₃SO₃H) solutions, and carefully transferred to Dialysis Tubing (MWCO3500) sealed with Spectra/Por Closures. The sealed Dialysis Tubing wasput into a 400 mL beaker with the solution of ammonium peroxydisulfate(184 mg) dissolved in 200 mL of 1M methanesulfonic acid (CH₃SO₃H)solution. The reaction was carried out at room temperature without anydisturbance. After 24 hours, precipitated dark-green polyaniline on thebottom of the beaker was collected and purified by dialysis withdeionized water (Dialysis Tubing, MWCO 12k-14k). Doped polyanilinenanofibers were dedoped by dialysis with 0.1 M NH₄OH_((aq)), and thenredoped by dialysis with 0.5M HCl_((aq)).

After purification with deionized water, the dark green polyanilinenanofibers in the dialysis tubing were diluted with deionized water andexamined by scanning electron microscopy (SEM, Philips XL-30 ESEM) andtransmission electron microscopy (TEM, Philips CM-200). As shown in FIG.1 a (SEM), and FIG. 1 b (TEM), the polyaniline precipitate presentsnanofibrous structures with diameters ranging from 30 nm to 80 nm,confirmed via TEM. FIG. 1 also shows that polyaniline nanofibers are ofthe interconnected, branched and networked morphology. However, afterdilution of the colloid suspension of polyaniline nanofibers with alarge amount of deionized water, some single polyaniline nanofibers canbe isolated from the agglomeration as shown in the inset SEM image (b)of FIG. 2. This indicates that this kind of polyaniline nanofibers canbe potentially used to fabricate nanoelectronic devices such asfield-effect devices, which are under investigation. In addition,polyaniline nanofibers formed within the Dialysis Tubing show similarnanostructures as these found outside.

FIG. 2 shows XRD patterns of polyaniline/CH₃SO₃ ⁻ nanofibers andpolyaniline/Cl⁻ nanofibers, and the electron diffraction of the singlepolyaniline/CH₃SO₃ ⁻ nanofiber (inset image a). Samples were dispersedin deionized water and deposited onto substrates. Electron diffractionwas taken from the polyaniline nanofiber.

One broad band observed with 2θ c.a. 24° shows that polyaniline/CH₃SO₃ ⁻nanofibers are disordered as shown in the pink line of FIG. 2. Electrondiffraction examining on the polyaniline/CH₃SO₃ ⁻ nanofiber shown in theinset image (a) of FIG. 2 also verifies the disordered structure of thepolyaniline/CH₃SO₃ ⁻ nanofiber. Moreover, after redoping with 0.5MHCl_((aq)), polyaniline/Cl⁻ nanofibers obtained appear similar in theXRD pattern with 2θ c.a. 24° as shown in FIG. 2. The inset image (b)taken by SEM, scale bar=1 μm.

UV/vis absorption spectra demonstrate that polyaniline nanofibersobtained have different absorption patterns corresponding to oxidationand reduction states as shown in FIG. 3. The absorption peaks for bothpolyaniline nanofiber salts and emeraldine base are consistent withpreviously reported results for bulk (nonfibrous) polyaniline. Thissupports the presence of the same chemical structure for nanofibrouspolyaniline and granular polyaniline. Furthermore, it is noted thatsolid-state FTIR spectrum of polyaniline nanofibers provides excellentagreement with previous studies of nonfibrous polyaniline as shown inFIG. 4. It is interesting that polyaniline nanofibers can be easilydispersed in deionized water to form homogeneous solutions as shown inthe inset image of FIG. 3.

Scanning electron micrograph (SEM) of polyaniline nanofibers depositedon Si-wafer substrates (a) dedoped with 0.1 M NH₄OH_((aq)) (b) redopedwith 0.5M HCl_((aq)) are shown in FIG. 5. In these micrographs,polyaniline nanofibers dispersed in deionized water were deposited onthe substrates for examination. The nanofibrous structures show nosignificant change after dedoping and redoping as shown in SEM's of FIG.5. This indicates that polyaniline nanofibers are very stable upon thetreatment with acid and base solutions.

The bulk conductivity of polyaniline nanofibers formed is obtained inthe range of ˜0.89 S/cm from the 4-probe DC measurement at roomtemperature, 24° C., for polyaniline/CH₃SO₃ nanofibers deposited on aglass slide to form a dark green film, and then four Au electrodesdeposited by thermal evaporation to form the contacts.

In summary, polyaniline nanofibers are produced via a novel technique.The branch and network nanostructures are demonstrated through bothscanning and transmission electron microscopy. The average diameters ofpolyaniline nanofibers range from 30 nm to 80 nm, confirmed viatransmission electron microscopy. UV/vis and FT-IR spectra ofpolyaniline nanofibers are consistent with the spectra of polyanilinepowders prepared by the traditional chemical synthesis. XRD and electrondiffraction indicate that polyaniline nanofibers formed are disordered.The nanofibrous morphology has no significant change withdoping/dedoping by the treatment with acid/base solutions. The roomtemperature bulk conductivity of polyaniline/CH₃SO₃ ⁻ nanofibers (σ˜0.89S/cm) is reported with 4-probe DC measurement.

Example 2 Using Low Concentration Bulk Polymerization

Aniline was dissolved in a small portion of 1M dopant acid solution, andcarefully transferred to a solution of ammonium peroxydisulfate (APS)dissolved in 1M dopant acid solution in the beaker. The reaction wascarried out at room temperature without any disturbance. After 24 hours,the dark-green polyaniline precipitate was collected to Dialysis tubing(MWCO 12k-14k), and then purified by dialysis with deionized water. Thevaried concentrations of aniline were used to study the formation ofpolyaniline nanofibers, including [aniline]=0.008M, 0.016M, 0.064M and0.128M. The molar ratio of aniline and APS was kept at 2:1. A variety ofacids were used, such as HCl, (1S)-(+)-10-camphorsulfonic acid (CSA),2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA), methanesulfonicacid (CH₃SO₃H), and HClO₄, etc. Polymerization was also investigated attemperature of 0 to 5° C. (ice bath) as well as under mechanicalstirring.

Samples deposited onto the Si-wafer substrates and then sputtered with athin layer of Au/Pd were used for studies of scanning electronmicroscopy (SEM, Philips XL-30 ESEM). Samples dispersed in deionizedwater were transferred to copper grids for the examination oftransmission electron microscopy (TEM, Philips CM-200 or Philips TF-20).UV/vis absorption was studied from UV/VIS/NIR Spectrometer (PERKIN ELMERLambda 19) employed the dispersion of samples in deionized water.

Polyaniline nanofibers were successfully obtained through thetraditional bulk polymerization without any aid of specific templates.FIG. 6 shows Transmission electron micrograph (TEM) of polyanilinenanofibers obtained in different dopant acids at [aniline]=0.008M and24° C. without mechanical stirring (a) CSA (scale bar=200 nm) (b)CH₃SO₃H (scale bar=200 nm) (c) HClO₄ (scale bar=500 nm). As shown inFIG. 6, TEM images obtained show nanofibrous structures of polyaniline.The sizes of polyaniline nanofibers vary depending on the synthesisconditions. As shown in FIG. 1( a), polyaniline nanofibers obtained fromCSA_((aq)) present smaller diameters ranging from 17 nm to 50 nm (basedon TEM measurement). The diameters of polyaniline nanofibers synthesizedin CH₃SO₃H_((aq)) rang from 42 nm to 70 nm (based on TEM measurement) asshown in FIG. 1( b). In addition, FIG. 1( c) shows larger diameters ofpolyaniline nanofibers obtained from HClO_(4(aq)) varying from 72 nm to230 nm (based on TEM measurement). Therefore, the diameters ofpolyaniline nanofibers can be controlled directly by the dopant acidsused.

FIG. 7 shows SEM images of polyaniline nanofibers synthesized indifferent dopant acids including CSA_((aq)) (a), HClO_(4(aq)) (b),HCl_((aq)) (c), AMPSA_((aq)) (d), and CH₃SO₃H_((aq)) (e) at an anilineinitial concentration of 0.008 M at 24° C. It is clear that the networkand branch morphologies of polyaniline nanofibers have no significantchange using different dopant acids for polymerization media. However,it is interesting that polyaniline nanofibers made in HClO_(4(aq))produce more linear nanofibrous structures than those made in others,albeit its average diameter is largest among the dopant acids used. Inaddition, polymerization temperature affects the formation ofpolyaniline nanofibers, as shown in FIG. 7( e) and FIG. 7( f).

Polyaniline/CH₃SO₃ ⁻ synthesized directly at 0° C. has smallernanofibrous structures than those fibers synthesized directly at 24° C.

During polymerization, the reaction kept still, i.e. withoutdisturbance, has a preference to the formation of polyaniline nanofibersover stirring. This is demonstrated in FIG. 8, which shows scanningelectron micrograph (SEM) of polyaniline/CH₃SO₃— nanofibers obtained in[aniline]=0.016M and [APS]=0.008M at 24° C. (a) without mechanicalstirring and (b) with mechanical stirring. As presented in FIG. 8,polyaniline nanofibers synthesized without mechanical stirring (FIG. 8(a)) are longer in length than those synthesized with mechanical stirring(FIG. 8 (b)). The concentration of aniline used is also an importantfactor to control the morphology of polyaniline nanofibers.

FIG. 9 shows SEM images of polyaniline nanofibers made in differentconcentrations of aniline at 24° C. without stirring. Theseconcentrations are (a) [aniline]=0.008M (b) [aniline]=0.016M (c)[aniline]=0.064M and (d) [aniline]=0.128M. Low concentration of anilinehas a tendency to form longer and less branched polyaniline nanofibersas shown in FIG. 4( a). However, the average size of polyanilinenanofibers obtained increases as the concentration of aniline decreases.

As mentioned above, the morphology of polyaniline nanofibers depends onthe conditions of polymerization. Briefly, FIG. 8( b), and FIG. 9(c)-(d) show that polymerization with mechanical stirring and highconcentration of aniline, respectively, are disadvantageous to theformation of polyaniline nanofibers. This correlates with conventionalbulk polymerization using high concentration of aniline with mechanicalstirring forming granular polyaniline.

Polyaniline/ClO₄ ⁻ nanofibers were dispersed in deionized water byvigorously shaking with a hand for the characterization of UV/visabsorption. FIG. 10 shows UV/vis absorption spectra of polyanilinenanofibers made by HClO₄ (aq). After purification by dialysis againstdeionized water, polyaniline nanofibers present three absorption peaksc.a. 338 nm, 430 nm and 960 nm with a free carrier tail as shown in thebright green line of FIG. 5. With adding one drop of dilute 70% w/wHClO_(4(aq)) to polyaniline nanofiber dispersion (bright green line),the absorption intensity of the peak at c.a. 338 nm decreases followingthe increase of the absorption intensity of the peak at c.a. 430 nm and960 with a free carrier tail as shown in the green line of FIG. 5. It ispossible that the purification of polyaniline nanofibers by dialysiswith deionized water results in removal of the dopant, ClO₄ ⁻, withinpolyaniline backbone to form partially doped polyaniline nanofibers.Furthermore, adding a drop of 30% w/w NH₄OH_((aq)) to polyaniline/ClO₄ ⁻nanofibers dispersion (green line) introduces the formation of anabsorption band c.a. 677 nm, simultaneously resulting in disappearanceof two absorption bands c.a. 430 nm and 960 nm with a free carrier tailas shown in the blue line of FIG. 5. Two strong absorption bands c.a.338 nm and 677 nm is attributed to the formation of emeraldine base. TheUV/vis absorption patterns of polyaniline nanofibers obtained areconsistent with previously reported results.

In summary, polyaniline nanofibers were successfully synthesized usingconventional bulk polymerization. With the appropriate control ofpolymerization conditions by using very dilute concentration of anilinewith modest, little or no disturbance, polyaniline will favorably formnanofibrous structures. Polymerization in different dopant acidsproduces similar morphology of polyaniline nanofibers. The size ofpolyaniline nanofibers is tunable under the appropriate selection ofdopant acids. UV/vis absorption shows that polyaniline nanofibers havethe same absorption patterns with previous as reported for nonfibrouspolyanilines.

Sensors based on polyaniline nanofiber network can be prepared by themethods described herein. For particular sensor shown in FIG. 11 thepolyaniline nanofiber network was assured to be fully doped and highlyconducting by exposure to 37% w/w aqueous HCl. The fiber network placedon a substrate was then exposed to vapor from a drop of 30% w/w aqueousNH₄OH placed several cm from the nanofiber network. The resistance ofthe nanofiber network increases over many orders of magnitude in a fewseconds. Substantial increase in the nanofiber network resistance occursin less then one second.

Field effect device based on polyaniline nanofibers in contact withsource and drain electrodes have been demonstrated. FIG. 12 is a graphshowing the conductance change of nanofibers upon applying gate voltagesin a field effect device based on polyaniline nanofibers. In thisexample, the nanofiber network with source and drain electrodes affixedis also in contact with a dielectric polymer which is in contact with agate electrode. Application of a gate voltage of less then 20 voltsresults in modulation of the electrical conductivity between source anddrain electrodes.

The exemplary embodiment has been described with reference to thepreferred embodiments. Obviously, modifications and alterations willoccur to others upon reading and understanding the preceding detaileddescription. It is intended that the exemplary embodiment be construedas including all such modifications and alterations insofar as they comewithin the scope of the appended claims or the equivalents thereof.

1. A process for forming nanofibers of polyaniline or a substitutedpolyaniline including the steps of: providing a first solutioncontaining aniline monomer or substituted aniline monomer; providing asecond solution containing an oxidant; providing a permeable membraneseparating said first and said second solutions, wherein said membranepermits at least one of said monomer and said oxidant to passtherethrough at a controlled rate; allowing at least one of said monomerand said oxidant to pass through said membrane to form a reactionsolution of monomer and oxidant; and polymerizing monomer in saidreaction solution to form polymer nanofibers; wherein at least one ofsaid first and second solutions further contains an acid.
 2. A processaccording to claim 1, wherein said permeable membrane comprises a porousmaterial.
 3. A process according to claim 1, wherein said permeablemembrane comprises at least one of regenerated cellulose, celluloseester and polyvinylidene difluoride.
 4. A process according to claim 3,wherein said permeable membrane comprises dialysis tubing.
 5. A processaccording to claim 3, wherein said regenerated cellulose membrane has amolecular weight cut-off of between 3500 and 60,000.
 6. A processaccording to claim 3, wherein said cellulose ester membrane has amolecular weight cut-off of between 100 and 300,000.
 7. A processaccording to claim 3, wherein said cellulose ester membrane has amolecular weight cut-off between 300 and 60,000.
 8. A process accordingto claim 3, wherein said polyvinylidene difuoride membrane has amolecular weight cut-off between 250,000 and 1,000,000.
 9. A processaccording to claim 1, wherein said substituted aniline monomercomprises:

wherein, R₁, R₂, R₃, R₄, R₅, and R₆ are independently selected from thegroup consisting of: hydrogen atom, alkyl, alkoxy, alkylsulfonyl,arylsulfonyl, halogen, alkoxycarbonyl, alkylhio, alkylsulfuryl,cycloalkyl, sulfonic, aryl or carboxylic substituted alkyl substituents.10. A process according to claim 9, wherein said substituted anilinemonomer comprises one or more of: 2-aminobenzenesulfonic acid,3-aminobenzenesulfonic acid, orthanilic acid, o-toluidine, m-toluidine,o-anisidine, m-anisidine, 2-fluoroaniline, 2-chloroaniline,2-bromoaniline, 2-iodoaniline, 3-fluoroaniline, 3-chloroaniline,3-bromoaniline, and 3-iodoaniline.
 11. A process according to claim 1,wherein at least one of said first and second solutions furthercomprises an acid comprising one or more of: hydrochloric acid, hydrogenbromide, sulfuric acid, perchloric acid, nitric acid, phosphoric acid,phosphonic acid, trifluoromethanesulphonic acid, toluenesulphonic acid,dodecylbenzenesulphonic acid, carboxylic acids, acetic acid,trichloroacetic acid, trifluoroacetic acid, formic acid,(1R)-(−)-10-camphorsulfonic acid, (1S)-(+)-10-camphorsulfonic acid,2-acrylamido-2-methyl-1-propanesulfonic acid, and methanesulfonic acid.12. A process according to claim 1, wherein said oxidative agentcomprises one or more of: ammonium peroxydisulfate, persulfated salts,perchlorated salts, chlorinated salt, iodonated salt, chlorinated iron,halogenated metal acids, fuming sulfuric acid, and ozone.
 13. A processaccording to claim 1 wherein use of said oxidizer results in reducedoxidant that remains in the resulting polymer nanofibers.
 14. A processaccording to claim 1, wherein a concentration of monomer in said firstsolution is from 0.001 M to pure monomer
 15. A process according toclaim 4, wherein said monomer is sealed inside said dialysis tubing anda concentration of said monomer in said tubing is about 0.335 M
 16. Aprocess according to claim 1, wherein said polymerization step isconducted at a temperature of from −10 to 60° C.
 17. A process accordingto claim 1, wherein the molar ratio of monomer to oxidant prior topolymerization is form 100:1 or to 0.01:1.
 18. A process according toclaim 1, wherein the molar ratio of monomer to oxidant prior topolymerization is from 4:1 to 0.25:1.
 19. A process according to claim1, wherein the molar ratio of monomer to oxidant prior to polymerizationis about 1:1.
 20. A process according to claim 1, further comprising thestep of dedoping the polymer nanofibers with a base to produce anon-electroconductive polymer product having an electrical conductivityless than 10⁻⁵ S/cm.
 21. A process according to claim 20, wherein saidbase comprises NH₄OH.
 22. A process according to claim 21, wherein aconcentration of said base is 0.1 M.
 23. A process according to claim20, further comprising the subsequent step of re-doping the polymerproduct with a suitable acid to produce an electroconductive polymerhaving an electrical conductivity greater than 10⁻⁵ S/cm.
 24. A processfor dedoping and redoping a polymer nanofiber formed according to theprocess of claim 1 using electrochemical oxidation and reductionreactions.
 25. A process according to claim 1, wherein said polymernanofibers have diameters ranging from 10 nm to 500 nm.
 26. A processaccording to claim 25, wherein said polymer nanofibers have diametersranging from 15 nm to 300 nm.
 27. A process according to claim 26,wherein said polymer nanofibers have diameters ranging from 30-80 nm.28. A process according to claim 11, wherein diameters of said polymernanofibers can be controlled by the selection of acid.
 29. A processaccording to claim 1, wherein the morphology of said polymer nanofiberscan be controlled by the temperature at which polymerization is carriedout.
 30. A process according to claim 1, wherein the morphology of saidpolymer nanofibers can be controlled by the addition of a surfactant toat least one of said first solution, said second solution, and saidreaction solution.
 31. A process according to claim 30, wherein thesurfactant is selected from among anion surfactants such as sodiumdodecylsulfate, cation surfactants such asdodecylltrimethylammoniumbromide, and nonionic surfactants such asTriton® X-100.
 32. A process according to claim 30, wherein thesurfactant concentration is in the range of 0.0001 M to 1 M.
 33. Aprocess for forming nanofibers of polyaniline or a substitutedpolyaniline including the steps of: providing an aqueous reactionmixture comprising an acid, aniline or substituted aniline monomer, andan oxidant, wherein said monomer and said oxidant are present in aconcentration of tens of millimoles or lower; and polymerizing saidmonomer to form polymer nanofibers.
 34. A process according to claim 33,wherein said steps are performed without stirring or agitation of thereaction mixture.
 35. A process according to claim 33, wherein saidsteps are performed with stirring and/or agitation of the reactionmixture.
 36. A process according to claim 33, wherein at least one ofsaid first and second solutions further comprises an acid comprising oneor more of: hydrochloric acid, hydrogen bromide, sulfuric acid,perchloric acid, nitric acid, phosphoric acid, phosphonic acid,trifluoromethanesulphonic acid, toluenesulphonic acid,dodecylbenzenesulphonic acid, carboxylic acids, acetic acid,trichloroacetic acid, trifluoroacetic acid, formic acid,(1R)-(−)-10-camphorsulfonic acid, (1S)-(+)-10-camphorsulfonic acid,2-acrylamido-2-methyl-1-propanesulfonic acid, and methanesulfonic acid.37. A process according to claim 36, wherein said polymer nanofibershave diameters ranging from 10 nm to 500 nm.
 38. A process according toclaim 37, wherein said polymer nanofibers have diameters ranging from 17nm to 230 nm.
 39. A process according to claim 33, wherein diameters ofsaid polymer nanofibers can be controlled by the selection of acid. 40.A process according to claim 33, wherein the morphology of said polymernanofibers can be controlled by the temperature at which polymerizationis carried out.
 41. A process according to claim 33, wherein themorphology of said polymer nanofibers can be controlled by the additionof a surfactant to at least one of said first solution, said secondsolution, and said reaction solution.
 42. A process according to claim41, wherein the surfactant is selected from among anion surfactants suchas sodium dodecylsulfate, cation surfactants such asdodecylltrimethylammoniumbromide, and nonionic surfactants such asTriton® X-100.
 43. A process according to claim 41, wherein thesurfactant concentration is in the range of 0.0001M to 1 M.
 44. Aprocess according to claim 33, wherein said substituted aniline monomercomprises:

wherein, R₁, R₂, R₃, R₄, R₅, and R₆ are independently selected from thegroup consisting of: hydrogen atom, alkyl, alkoxy, alkylsulfonyl,arylsulfonyl, halogen, alkoxycarbonyl, alkylhio, alkylsulfuryl,cycloalkyl, sulfonic, aryl or carboxylic substituted alkyl substituents.45. A process according to claim 44, wherein said substituted anilinemonomer comprises one or more of: 2-aminobenzenesulfonic acid,3-aminobenzenesulfonic acid, orthanilic acid, o-toluidine, m-toluidine,o-anisidine, m-anisidine, 2-fluoroaniline, 2-chloroaniline,2-bromoaniline, 2-iodoaniline, 3-fluoroaniline, 3-chloroaniline,3-bromoaniline, and 3-iodoaniline.
 46. A process according to claim 33,wherein said oxidative agent comprises one or more of: ammoniumperoxydisulfate, persulfated salts, perchlorated salts, chlorinatedsalt, iodonated salt, chlorinated iron, halogenated metal acids, fumingsulfuric acid, and ozone.
 47. A process according to claim 33, whereinsaid polymerization step is conducted at a temperature of from −10 to60° C.
 48. A process according to claim 33, wherein a concentration ofmonomer in said reaction solution is from 0.001 to 0.2 M.
 49. A processaccording to claim 48, wherein said concentration is about 0.008 M. 50.A process according to claim 33, further comprising the step of dedopingthe polymer nanofibers with a base to produce a non-electroconductivepolymer product having an electrical conductivity less than 10⁻⁵ S/cm.51. A process according to claim 50, wherein said base comprises NH₄OH.52. A process according to claim 50, wherein a concentration of saidbase is 0.1 M.
 53. A process according to claim 50, further comprisingthe subsequent step of re-doping the polymer product with a suitableacid to produce an electroconductive polymer having an electricalconductivity greater than 10⁻⁵ S/cm.
 54. A process for dedoping andredoping a polymer nanofiber formed according to the process of claim 33using electrochemical oxidation and reduction reactions.
 55. A processaccording to claim 33, wherein the molar ratio of monomer to oxidantprior to polymerization is from 50:1 or to 0.02:1.
 56. A processaccording to claim 55, wherein the molar ratio of monomer to oxidantprior to polymerization is from 4:1 to 1:1.
 57. A process according toclaim 56, wherein the molar ratio of monomer to oxidant prior topolymerization is about 1.5:1.
 58. A field effect device including anactive channel comprising a polyaniline or substituted polyanilinenanofiber network in contact with a source electrode and a drainelectrode.
 59. A process for forming nanofibers of polyaniline or asubstituted polyaniline including the steps of: providing a reactionmixture comprising an acid, aniline or substituted aniline monomer,dimer or oligomer having up to eight repeating monomer units and anoxidant, wherein said monomer and said oxidant are present in aconcentration of tens of millimoles or lower; and polymerizing saidmonomer to form polymer nanofibers.
 60. A process for forming nanofibersof polyaniline or a substituted polyaniline including the steps of:providing a first solution containing a reactive species comprising atleast one of an aniline or substituted aniline monomer, dimer oroligomer having up to eight repeating monomer units; providing a secondaqueous solution containing an oxidant; providing a permeable membraneseparating said first and said second solutions, wherein said membranepermits at least one of said reactive species and said oxidant to passtherethrough at a controlled rate; allowing at least one of saidreactive species and said oxidant to pass through said membrane to forma reaction solution of reactive species and oxidant; and polymerizingsaid reactive species to form polymer nanofibers; wherein at least oneof said first and second solutions further contains an acid.